This invention relates to high performance magnetic recording apparatus. More particularly, the invention relates to such apparatus incorporating a particularly advantageous combination of recording medium and recording/playback head for recording magnetic impulses perpendicular to the surface of the recording medium.
Conventional high performance recording devices or subsystems, particularly those used in digital recording, employ a longitudinal recording format in which the magnetization resides in the plane of the recording medium. In order to obtain higher performance, in the form of a greater linear density of recording data, the recording medium is required to be thin, generally less than 20 microinches, and to exhibit a high coercive force, typically greater than 350 oersteds. One of the major limitations to the linear recording density is determined by the in-plane demagnetizing field associated with the recorded bit. This demagnetizing field increases as the bit size decreases, i.e., with increasing linear density, and ultimately approaches a value of 4.pi. M.sub.r (where M.sub.r is the remanent magnetization) which limits the linear density. Consequently, in order to attain high linear density, the coercive force of the material must be increased and/or the remanence-thickness product of the material decreased. However, the increase in coercive force is restricted by the availability of the recording head field, which is limited by the saturation of the recording head pole pieces when high coercive force materials are used. The performance is also limited by a loss in signal amplitude as the remanence-thickness product is decreased. Thus, when utilizing conventional magnetic recording systems, such as magnetic disc recorders, practical limits of operation are reached as the recorded wavelength decreases (to achieve high linear recording density), since the demagnetizing field would increase to 4.pi. M.sub.r, which would exceed the coercive force of the medium. However, as this condition is approached the media demagnetizes, reducing the amplitude of the magnetic signal.
In order to maintain the demagnetizing field below the coercive force of the medium it has been proposed by several researchers, notably by Iwasaki and Nakamura in their papers and by Suzuki and Akuta in U.S. Pat. No. 4,034,410 to record magnetic bits of data perpendicular to the plane of a recording disc, thereby avoiding the demagnetizing characteristics in the plane. In this perpendicular recording mode the demagnetizing field decreases to zero with decreasing wavelength, due to partial cancellation arising from fields from neighboring recording bit cells. Thus, a thinner medium with a high coercive force is not necessary since the demagnetizing field is not a factor. In addition, the signal-to-noise ratio can be improved, since the apparatus can utilize a thicker medium, which provides a greater signal due to the fact that the remanent magnetic moment per bit is much larger than in longitudinal recording. However, in order to obtain the advantages offered by perpendicular recording, not only must the recording medium exhibit an easy axis perpendicular to the medium plane, but also a magnetic head arrangement must be provided that produces a magnetic field highly perpendicular to the surface of the medium. This has been accomplished in the prior art, such as by Iwasaki et al. and by Suzuki et al., by the use of a recording head having an asymetric structure with the main pole in contact with or in close proximity to the recording medium, as on the top side of a recording disc, to record signals. The other pole of that head, an auxiliary pole, is very large in size compared with the main pole and is positioned on the other side of the recording medium, e.g. below the recording disc. In this prior art the auxiliary pole is energized by a winding around it, and the main pole is magnetized from its pole tip. This arrangement causes the recording magnetic field to extend from the main pole to the auxiliary pole, passing through the recording medium substantially perpendicular to its surface. While this arrangement does provide one form of recording perpendicular to the medium, it possesses several significant disadvantages. One such disadvantage is the requirement of two poles, one on each opposed side of the recording medium, and another disadvantage is that of the requirement of a playback head separate and different from the recording head, a requirement brought on by the inadequate playback efficiency of the recording head at short wavelengths, as described by Iwasaki et al. in IEEE Transactions on Magnetics, Vol. Mag-13, No. 5, pages 1272-1277, September 1977.